Method for preparing chromic acid



July 25, 1961 R. E. 'MARLATT EFAL 2,993,756

METHOD FOR PREPARING CHROMIC ACID,

Filed June 26, 1959 E030] EmIFOE OwI 2 M0310 EE-CO2 INVENTORY RALPH E. MARLATT TOM s. PERRIN ROBERT G. BANNER 7km ;5' W

ATTORNEY mmxq l 0P n05 EE m 06 rllilll United This invention relates to improvements in the method of making chromic acid and, more particularly relates to improvements in the method of making chromic acid by the reaction of an aqueous solution of an alkali metal bichromate and sulfuric acid.

In the manufacture of chromic acid, two processes are presently in use. In the conventional or so called dry process, solid alkali metal bichromate is fused With 20% oleum to form two liquid layers, the top layer consisting mainly of alkali metal bisulfate and the bottom layer being chromic acid. Inasmuch, as in this process the weight of the top layer of alkali metal bisulfate is about 160% greater than that of the chromic acid, difiiculties are encountered in effecting a separation of the two layers as well as recovering the acid values of the bisulfate layer, to say nothing of disposal problems encountered with this large mass of material. As a result of these dilficulties, a second process for producing chromic acid has been developed, which process is commonly-known as the wet process.

In this process, a 70% aqueous solution of alkali metal bichromate is reacted with 93% sulfuric acid at a temperature between about 50 and 75 C. Crude chromic acid crystals are precipitated from this reaction mixture and removed therefrom by filtration. The crude crystals are then fused so as to form two layers, the upper layer being bisulfate and the lower layer being the chromic acid, the weight of which upper layer is only about 12% as great as that of the chromic acid layer. However, the economic feasibility of this process depends upon utilizing the filtrate or mother liquor obtained in the separation of the crude chromic acid crystals, which filtrate contains a high percentage of acid values in the form of sulfuric acid and alkali metal bisulfate as well as chromic acid values.

In the past, it has generally been the practice to return this filtrate or mother liquor to a process for making alkali metal bichromate, wherein the acid values in the mother liquor are used to convert alkali metal chromate into the bichromate, the bichromate being recovered as the product of the process after precipitating sodium sulfate therefrom. It will be appreciated that such use of the filtrate or mother liquor from the chromic acid process unavoidably ties this process to a process for producing alkali metal bichromate. Thus, the chromic acid capacity of such a process is limited by the amount of filtrate or mother liquor which can be utilized in the bichromate process, which in turn depends upon the available market for the alkali metal bichromate. This close tie-in of the chromic acid process and the alkali metal bichromate process is particularly distressing in that any increase in the demand for the chromic acid without a corresponding increase in the demand for alkali metal bichromate necessitates either an over production of hichromate so as to utilize the acid values in the filtrate from the chromic acid process or discarding the filtrate from the chromic acid process, thereby losing the acid values contained therein as well as losing the economic advantages obtained by the utilization of this filtrate. it is believed to be quite obvious that neither of these expedients is particularly attractive from an economic standpoint in a commercial operation.

In an effort to reduce this dependency of the chromic acid process on an alkali metal bichromate process, it

2,993,756 Patented July 25, 1961 the has been proposed to utilize the chromic acid process filtrate for the manufacture of tanning liquors. However, inasmuch as the cost of transporting this filtrate or mother liquor for any appreciable distance would necessitate pricing the tanning liquor thus produced at an uncompetitive level, it is apparent that this proposal is feasible only where the facilities for producing the tanning liquor are available in an area closely adjacent to that in which the chromic acid is produced. In view of this, the practice of utilizing the filtrate of the chromic acid process in making tanning liquors has not been widely adopted.

A further proposal for utilizing the filtrate from the chromic acid process has been to concentrate this filtrate so that it may be reused in the chromic acid process without adding an excessive amount of water to the process or causing a build-up of alkali metal bisulfate. However, it has been found that in concentrating the filtrate an excessive amount of the chromium values contained therein are converted from the hexavalent chromium state to trivalent chromium and, thus, are not suitable for reuse in the chromic acid process. It is seen that up to the present time there has been no satisfactory substitute for the utilization of the chromic acid filtrate in an alkali metal bichromate process, which use is itself not wholly satisfactory because of the necessary dependency of the chromic acid process on the process for making alkali metal bichromate.

It is, therefore, an object of the present invention to provide an improved method for producing chromic acid whereby the present production in a chromic acid process can be increased without the necessity of increasing the production in an alkali metal bichromate process.

Another object of the present invention is to provide an improved process for making chromic acid wherein the chromic acid values in the chromic acid process filtrate are utilized in the chromic acid process.

A further object of the present invention is to provide an improved chromic acid process wherein the acid values in the process filtrate are recovered as solid alkali metal bisulfate.

These and other objects will become apparent to those skilled in the art from the description of the invention which follows.

The drawing which is attached hereto and forms a part hereof is a schematic flow diagram illustrating one embodiment of the present invention.

In the description of the invention and the claims which follow, the term 'alkali metal bichromate is meant to refer to the bichromates of sodium, potassium, lithium, cesium and rubidium. However, because of its low cost and ready availability, sodium bichromate is the preferred material, and hence primary reference will be made thereto.

In the preparation of chromic acid by the reaction of an aqueous solution of an alkali metal bichromate and sulfuric acid, the method of the present invention envisions reacting an aqueous solution of an alkali metal bichromate, containing about 88% by weight alkali metal hichromate with sulfuric acid of a concentration of about filtering the reaction mixture to remove the crystals of crude chromic acid, which crystals are then processed to obtain pure chromic acid, cooling the filtrate to a temperature sufiiciently low to crystallize alkali metal bisnlfate, filtering the thus-formed slurry to recover crude alkali metal bisulfate, and returning the mother liquor or filtrate obtained from this filtration back to the initial reactor to be used in making up a new reaction charge to form chromic acid. The crude alkali metal bisulfate crystals obtained in such process can, if desired, be utilized in a process for making alkali metal bichromate wherein the acid value of these crystals is used to convert alkali metal chromate liquors to alkali metal bichromate liquors, alternatively, the solid, crude alkali metal bisulfate may be recovered and used in either the crude or purified form in any other process where its use is desired.

More specifically, in the practice of the present method, an aqueous solution of sodium bichromate containing about 88% by weight sodium bichromate is reacted with sulfuric acid having a concentration of about 100%, and the return mother liquor obtained from a previous production run of chromic acid. The temperature of the reaction is controlled so as to preferably not be in excess of about 75 0, although higher temperatures can be used. The proportion of sulfuric acid to sodium bichromate in the total reaction mixture is within the range of 3.0 to 4.5 mols. of sulfuric acid to 1.0 mol. of

Na CI'2O7 about 3.5 mols. of sulfuric acid to 1 mol. of

Na C1' O- being the preferred ratio. At this reaction temperature and concentration of reactants, the reaction is substantially instantaneous to form a slurry containing crystals of crude chromic acid. This slurry is filtered to recover the crystals of crude chromic acid, which crystals are then purified by melting, and are then flaked and packed for shipping. The filtrate recovered from the filtration of the slurry containing the crude chromic acid crystals is cooled to a temperature sufiiciently low to promote the crystallization of sodium bisulfate therefrom. This temperature is generally about 20 to 25 C. Once the sodium bisulfate has crystallized so as to form a slurry containing crystals of sodium bisulfate, the slurry is filtered and the crude sodium bisulfate crystals are recovered. These crystals may be utilized without further purification, as for example, by returning them to a process for making sodium bichromate wherein the acid values in these crystals are utilized to convert sodium chromate solution to sodium bichromate. If desired, these crude sodium bisulfate ciystals may be purified and sold as such or they can be converted to sodium sulfate which in turn can be sold. The mother liquor from the second filtration, wherein the crude sodium bisulfate crystals are recovered, is returned to the reactor, wherein it is admixed with 88% sodium bichromate solution and 100% sulfuric acid so as to form additional chromic acd.

By operating in accordance with the above procedure, the amount of material containing acid values which must be utilized in order to enjoy the fullest economical advantages of this process are reduced by about one-half, thus making it possible to double the chromic acid production of the present process without necessitating any additional increase in the sodium bichromate process which utilizes these acid values. However, it will be appreciated, that in order for the present process to operate effectively, the amount of mother liquor produced, which is utilized as part of the reaction charge in making the chromic acid, as well as the composition of this mother liquor, must be such that the sulfuric acid to sodium bichromate ratio in the total reaction mixure is approximately 3.5 mols. to 1 mol. Additionally, it is obvious that the percent solids or crystals in the reaction mixture must be such as to be readily workable, so that problems in handling and filtration are not encountered. Inasmuch as these factors, i.e., the amount and composition of the mother liquor and the reaction mixture viscosity are effected by the acidity and the water content of the reaction mixture, it has been found that by utilizing a sodium bichromate liquor having a sodium bichromate content of about 88% and 100% sulfuric acid in conjunction with the return mother liquor the desired consistency of the reaction mixture will be maintained, and the composition and volume of the mother liquor produced will remain consistent, so that no variations in these concentrations are necessary.

It is to be appreciated that in the start-up of the present process, when there is no return mother liquor available to be used in making up the reaction charge, the reaction charge will consist of an aqueous sodium bichromate liquor having a sodium bichromate content of about 70% by weight and about 93% to 95%sulfuric acid. The process is then carried out as described above, i.e., the crude chromic acid is recovered by filtration, the filtrate cooled to crystallize the sodium bisulfate crystals which are then removed by filtration and the mother liquor is returned to the reactor to be used in making up a new reaction charge. At this point, i.e., once return mother liquor is available, the concentrations of the sodium bichromate liquor and sulfuric acid which are added to the reactor are increased to 88% by weight and 100% respectively. Thereafter, as long as there is return mother liquor available, no further variations in the concentrations of the bichromate liquor and sulfuric acid will be necessary.

By using an sodium bichromate liquor and 100% sulfuric acid in the present process rather than 70% bichromate liquor and 93% to 95% sulfuric acid as has been previously used, it is found that there is no build-up of water in the system. It is, thus, possible to utilize the bichromate values of the return mother liquor in the chromic acid process while maintaining both the ratio of acid to bichromate in the reaction mixture and the amount of water therein at the desired level without the necessity for evaporation. This return mother liquor has been found to have the following average composi tion in percent by weight:

Referring now to the drawing which is attached hereto and forms a part hereof, bichromate liquor having a sodium bichromate concentration of about 88% by weight, sulfuric acid and the return mother liquor having a composition as set forth above are added to the reactor. Within the reactor, chromic acid crystallizes from the reaction mixture, which is maintained at a temperature between about 50 and 75 C., and the slurry of chromic acid crystals is withdrawn from the reactor. From the reactor, this slurry of chromic acid crystals passes to a filter wherein the crude chromic acid crystals are recovered. These crude chromic acid crystals pass to a melting pot from which pure chromic acid is recovered and sent to a flaker. The filtrate, from which the crude chromic acid crystals have been removed, after leaving the filter, passes to a crystallizer wherein it is cooled to a temperature which will promote the crystallization of sodium bisulfate therefrom. The temperature within the crystallizer is suitably maintained at about 20 to 25 C. Once the crystals of sodium bisulfate are formed, they are removed from the crystallizer as a slurry, which slurry passes to a filter wherein the crude crystals of sodium bisulfate are recovered. The mother liquor from this second filtration passes from the filter through a surge tank and returned to the reactor where it is used, along with the 88% bichromate and 100% sulfuric acid, in making up a new reaction charge.

The crude sodium bisulfate crystals recovered in this process may, if desired, be purified and sold as commercial sodium bisulfate or it may be converted to sodium sulfate and sold. Preferably, however, these crystals are returned to a process for making sodium bichromate wherein the acid values contained in these crystals are utilized in converting sodium chromate liquor to sodium bichromate liquor. It should be further appreciated that although the 88% bichromate liquor used in making up the reaction charge in the present process may come from any suitable source, it has been found to be particularly convenient to obtain this liquor from the sodium bichromate process wherein it is readily available. However, regardless of the source of the sodium bichromate liquor, it has been found to be necessary, due to its freezing point to heat the supply lines of the bichromate liquor to prevent it from freezing up in these lines.

In actual operation, based on a desired production of 20 tons per day of chromic acid, a total of 29.38 tons of 100% sulfuric acid, 38.22 tons of 88% sodium bichromate liquor and 32.6 tons per day of return mother liquor are added to the reactor. From the reactor, the slurry of chromic acid crystals are sent to the filter at the rate of 653.9 gallons per hour. From the filter, a total of 27.94 tons per day of crude chromic acid are recovered, which crude chromic acid is passed to a melting pot from which a total of 20 tons per day of chromic acid are recovered. The filtrate from which the crude chromic acid has been recovered, is sent to a crystallizer at the rate of 416 gallons per hour. Within the crystallizer, the temperature of this filtrate is reduced to 23 C. so as to crystallize sodium bisulfate therefrom. From the crystallizer a slurry of sodium bisulfate is passed through a filter wherein a total of 36.80 tons per day of crude sodium bisulfate is recovered. This quantity of crude sodium bisulfate represents an acid equivalent of 14.14 tons per day sulfuric acid. The mother liquor from this second filtration is passed through a surge tank and then returned to the reactor at the rate of about 32.6 tons per day, wherein it is admixed with 100% sulfuric acid and 88% sodium bichromate liquor to be used in producing additional chromic acid.

From the above, it is seen that in the subject improved process for producing chromic acid, for a 20 tons per day chromic acid production, the acid equivalent of the material returned to a sodium bichromate process is only 14.14 tons per day sulfuric acid. In contrast, when operating the process using 70% sodium bichromate liquor and 95% sulfuric acid, without crystallizing sodium bisulfate from the first filter liquor, the acid equivalent in the material returned to the sodium bichromate process is about 14 tons per day sulfuric acid for a chromic acid production of tons per day. It is, thus, evident that by the method of the present invention the daily production of chromic acid can be doubled without materially increasing the amount of acid equivalent material returned to a sodium bichromate process and hence without the need for a corresponding increase in sodium bichromate production to utilize this material.

It will, of course, be appreciated that the present improved process offers an additional advantage in that, as the acid equivalent material is recovered in the form of crude crystals of sodium bisulfate, rather than a solution, all or any portion of these crude crystals can be readily disposed of in a process other than one for making sodium bichromate. In view of this, it is no longer necessary that the process for producing chromic acid be operated in conjunction with one for producing sodium bichromate, although such dual operation is advantageous and for this reason is generally preferred.

While there have been described various embodiments of the invention, the methods described are not intended to be understood as limiting the scope of the invention as it is realized that changes therewithin are possible, and it is further intended that each element recited in any of the following claims is to be understood as referring to all equivalent elements for accomplishing substantially the same results in substantially the same or equivalent manner, it being intended to cover the invention broadly in whatever form its principle may be utilized.

What is claimed is:

1. In the method of preparing chromic acid by the reaction of an aqueous solution of an alkali metal bichromate having a concentration of about 70% by weight alkali metal bichromate and sulfuric acid having a concentration between about 93 and wherein a slurry of crude chromic acid crystals is formed, the slurry fi-ltered to recover the crude crystals of chromic acid, the filtrate which remains after the crude chromic acid crystals are recovered is cooled to crystallize crude alkali metal bisulfate, and the crude alkali metal bisulfate is separated therefrom, the improvement which comprises reacting an aqueous solution of alkali metal bichromate having a concentration of about 88% by weight alkali metal bichromate and sulfuric acid having a concentration of about with the mother liquor from which the crude alkali metal bisulfate is separated, the sulfuric acid and alkali metal bichromate being added in an amount such that the total reaction mixture has a ratio within the range of 3.04.5 mols of sulfuric acid to 1.0 mol of alkali metal bichromate, to a slurry of crude chromic acid crystals, separating crude chromic acid crystals from the slurry, cooling the filtrate recovered in the separation of the chromic acid crystals from the slurry to a temperature sufiiciently low to effect crystallization of crude alkali metal bisulfate from the filtrate, separat ing the thus-formed crude alkali metal bisulfate and recycling the mother liquor recovered from this separation for use in making up a new reaction charge for the preparation of more chromic acid.

2. The method as claimed in claim 1 wherein the alkali metal bichromate is sodium bichromate and the alkali metal bisulfate is sodium bisulfate.

3. The method as claimed in claim 2 wherein the filtrate recovered in separating the crude chromic acid crystals is cooled to a temperature which is within the range of 20-25 C.

4. A continuous. method of preparing chromic acid comprising reacting an aqueous mixture of an alkali metal bichromate having a concentration of about 70% by weight alkali metal bicromate and sulfuric acid having a concentration between about 93% and 95%, in a ratio of 3.0 to 4.5 mols of sulfuric acid to 1.0 mol. of alkali metal bichromate, to form a slurry of chromic acid crystals, separating the crude chromic acid crystals from said slurry and purifying them to recover pure chromic acid, cooling the filtrate from which the chromic acid crystals have been separated to a temperature sutficiently low to effect the crystallization of alkali metal bisulfate therefrom, separating the thus-formed crude crystals of alkali metal bisulfate, recycling the mother liquor from which the crude crystals of alkali metal bisulfate have been separated to the reactor to which the bichromate solution and sulfuric acid are added and thereafter adding to said reactor an aqueous solution of alkali metal bichromate having a concentration of about 88% by weight alkali metal bichromate and sulfuric acid having a concentration of about 100% to react with said recycled mother liquor so as to form more chromic acid, the sulfuric acid and alkali metal bichromate being added in an amount such that the total reaction mixture has a ratio within the range of 3.0 to 4.5 mols of sulfuric acid to 1.0 mol of alkali metal bichromate.

5. A continuous method of preparing chromic acid comprising reacting an aqueous solution of sodium bichromate having a concentration of about 70% by weight sodium bichromate and sulfuric acid having a concentration between about 93% and 95%, in a ratio of 3.0 to 4.5 mols of sulfuric acid to 1.0 mol of sodium bichromate, to form a slurry of chromic acid crystals, separating the crude chromic acid crystals from said slurry and purifying them to recover pure chromic acid, cooling the filtnate from which the chromic acid crystals have been separated to a temperature sufficiently low to effect the crystallization of sodium bisulfate therefrom, separating the thus-formed crude crystals of sodium bisulfate, recycling the mother liquor from which the crude crystals of sodium bisulfate have been separated to the reactor to which the bichromate solution and sulfuric acid are added and thereafter adding to said reactor an aqueone solution of sodium bichromate having a concentration of about 88% by weight sodium bichromate and sulfuric acid having a concentration of about 100% to react with said recycled mother liquor so as to form more chromic acid, said sulfuric acid and sodium bichromate being added in amounts such that the total reaction mixture has a ratio Within the range of 3.0 to 4.5 mols of sulfuric acid to 1.0 mol of sodium bichromate.

6. The method as claimed in claim 5 wherein the filtrate from which the crude chromic acid crystals have been separated is cooled to a temperature which is Within the range of about 20-25 C.

References Cited in the file of this patent UNITED STATES PATENTS UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2 993 756 July 25 1961 Ralph E. Marlatt et 1."

It is hereby certified that error appears f in the above numbered pat ent requiring correction and that the said Letters *Periaent should read as corrected below.

Column 3, line 47', for "acd." read acid. line 62 for "mixure" read mixture column 4, line 22 for "80%" read 88% column 6 line 19, after "to" insert form Signed and sealed this 26th day of December 1961.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents USCOMM-DC 

1. IN THE METHOD OF PREPARING CHROMIC ACID BY THE REACTION OF AN AQUEOUS SOLUTION OF AN ALKALI METAL BICHROMATE HAVING A CONCENTRATION OF ABOUT 70% BY WEIGHT ALKALI METAL BICHROMATE AND SULFURIC ACID HAVING CONCENTRATION BETWEEN ABOUT 93 AND 95% WHEREIN A SLURRY OF CRUDE CHROMIC ACID CRYSTALS IS FORMED, THE SLURRY FILTERED TO RECOVER THE CRUDE CHROMIC ACID, THE FILTRATE WHICH REMAINS AFTER THE CUDE CHROMIC ACID CRYSTALS ARE RECOVERED IN COOLED TO CRYSTALLIZE CRUDE ALKALI METAL BISULFATE, AND THE CRUDE ALKALI METAL BISULFATE IS SEPARATED THEREFROM, THE IMPROVEMENT WHICH COMPRISES REACTING AN AQUEOUS SOLUTION OF ALKALI METAL BISULFATE IS HAVING A CONCENTRATION OF ABOUT 88% BY WEIGHT ALKALI METAL BICHROMATE AND SULFURIC ACID HAVING A CONCENTRATION OF ABOUT 100% WITH THE MOTHER LIQUOR FROM SHICH THE CRUDE ALKALI METAL BISULFATE IS SEPARATED, THE SULFURIC ACID AND ALKALI METAL BICHROMATE BEING ADDED IN AN AMOUNT SUCH THAT THE TOTAL BICHROMATE BEING ADDED IN AN WITHIN THE RANGE OF 3.0-4.5 MOLS OF SULFURIC ACID TO 1.0 MOL OF ALKALI METAL BICHROMATE, TO A SLURRY OF CRUDE CHROMIC ACID CRYSTALS, SEPARATING CRUDE CHROMIC ACID CRYSTALS FROM THE SLURRY, COOLING THE FILTRATE RECOVERED IN THE SEPARATION OF THE CHROMIC ACID CRYSTALS FROM THE SLURRY TO A TEMPERATURE SUFFICIENTLY LOW TO EFFECT CRYSTALLIZATION OF CRUDE ALKALI METAL BISULFATE FROM THE FILTRATE, SEPARATING THE THUS-FORMED CRUDE ALKALI METAL BISULFATE AND DRAZINE. RECYCLING THE MOTHER LIQUOR RECOVERED FROM THIS SEPARATION FROM THE USE IN MAING UP A NEW REACTION CHARGE FOR THE PREPARATION OF MORE CHROMIC ACID. 